diff --git a/mrbgems/mruby-sprintf/src/sprintf.c b/mrbgems/mruby-sprintf/src/sprintf.c
index a562bdac1..41c84eff1 100644
--- a/mrbgems/mruby-sprintf/src/sprintf.c
+++ b/mrbgems/mruby-sprintf/src/sprintf.c
@@ -153,12 +153,14 @@ fmt_float(char *buf, size_t buf_size, char fmt, int flags, mrb_int width, int pr
 }
 #endif
 
-#define CHECK(l) do {                           \
-  while ((l) >= bsiz - blen) {\
-    if (bsiz > MRB_INT_MAX/2) mrb_raise(mrb, E_ARGUMENT_ERROR, "too big specifier"); \
-    bsiz*=2;\
+#define CHECK(l) do { \
+  if (blen+(l) >= bsiz) {\
+    while (blen+(l) >= bsiz) {\
+      if (bsiz > MRB_INT_MAX/2) mrb_raise(mrb, E_ARGUMENT_ERROR, "too big specifier");\
+      bsiz*=2;\
+    }\
+    mrb_str_resize(mrb, result, bsiz);\
   }\
-  mrb_str_resize(mrb, result, bsiz);\
   buf = RSTRING_PTR(result);\
 } while (0)